Slit scanned electrostatic printing



Dec. 16, 1969 J. DAY

SLIT SCANNED ELECTROSTATIC PRINTING 2 Sheets-Sheet 1 Filed May 8, 1968MOTOR DELWEQY DEVSCLE.

9 POWDER sowzca //v VENTOI? Jaw DA y MOTOR A ro/2N5 Y5 Dec. 16, 1969Filed May 8, 1968 MOTOR POWDER SUPPLY J. DAY

SLIT SCANNED ELECTROSTATIQPRINTING PoTENnAL 2 Sheets-Sheet 2 AnaPRE$$uRE soulzca LOW A\ R PRESURE SOURCE LOW A? PRESSURE SOLA RCE PcwDERSUPPLY HIGH AUZ PRESSURE OURCE Jomv. DA y A 77ORNE vs 3,484,022 SLITSCANNED ELECTROSTATIC PRINTING John Day, Mountain View, Calif., assignorto Monsanto Graphic Systems, Inc., St. Louis, Mo., a corporation ofDelaware Filed May 8, 1968, Ser. No. 727,618 Int. Cl. B67d 5/06, 5/54;B05b 5/02 US. Cl. 22276 8 Claims ABSTRACT OF THE DISCLOSURE BACKGROUNDOF THE INVENTION This invention relates to electrostatic printing andmore particularly to improved arrangements for feeding electroscopicpowder for an electrostatic printing system.

In Patent No. 3,081,698 to Childress et al., an electrostatic printingsystem has been described wherein electroscopic powder is applied to onesurface of an image screen, which lets the powder through only in areaswhich are in the form of a desired image. The powder which passesthrough the screen then enters into an electric field which carries itto a substrate. The powder is thereafter caused to adhere to thesubstrate. The structures for applying the powder to the image screentake a number of different forms. Regardless of the form however, it isdesirable to apply the powder in as uniform a manner as possible, to theimage screen in order to obtain a uniform powder deposit on thesubstrate. Also, in order to enable the printing to occur over areasonably short interval, which is especially the case where the imagescreen is in motion, it is desirable to apply the powder to the imagescreen in a reasonably dense form, to obtain the best possible coverageon the substrate in the shortest time.

A number of different arrangements for applying the electroscopic powderto the image screen are known, including the use of a brush or a belt,or a powder cloud.

OBJECTS AND SUMMARY OF THE INVENTION tem.

Yet another object of this invention is to provide a powder cloudgenerator which generates a uniform and dense powder cloud.

Still another object of this invention is the provision of a novel anduseful powder cloud generator for an electrostatic printing system.

The foregoing and other objects of the invention may be achieved bycirculating a powder cloud in a closed loop path within a chamber whichhas a slot in one wall. Opposite to the slot is a plate or electrode.The powder cloud circulates between the slot and the electrode whereby,when a potential of the proper polarity is applied to the electrode,powder is forced out of the slot in response to the electric field whichis established. In one embodiment of the invention, the powder iscirculated by a rotating brush and the electrode is disposed centrallyof the chamber whereby, as the brush rotates a powder cloud is formedwhich moves in a path around the plate. In another embodiment, asubstantially circular chamber is provided into which the powder cloudis introduced. Also, air is introduced for the purpose of driving thepowder cloud around A United States Patent O 3,484,022 Patented Dec. 16,1969 the chamber. In one wall of the chamber is an electrode. In theopposite wall there is an opening or slot through which the powder cloudis caused to move, upon the application of the proper potential to theelectrode. Provision is made to remove the introduced air sinceotherwise the buildup of air pressure would cause the powder to leavethe chamber in an uncontrolled fashion.

The novel features of the invention are set forth with particularity inthe appended claims. The invention will best be understood from thefollowing description when read in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is an isometric view of anelectrostatic printing system showing the manner of the application ofthis invention.

FIGURE 2 is an isometric view of a powder delivery device in accordancewith this invention.

FIGURE 3 illustrates another powder delivery device in accordance withthis invention.

FIGURE 4 illustrates yet another powder delivery device in accordancewith this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGURE 1 illustrates anarrangement of an electrostatic printing system of the type which canemploy this invention. An image screen 10 is supported stretched betweentwo rollers respectively 12, 14. One of the rollers is driven by a motor16 whereby the screen can move past a location at which printing occurs.An electroscopic powder delivery device 18 applies powder to one side ofthe image screen 10. The powder passes therethrough under the influenceof an electric field which is established between the image screen and aplate electrode 20. The powder delivery device is connected to a sourceof potential 22, as is the image screen and the electrode 20. As aresult a powder delivery field is established which causes the powder tomove from the powder delivery device up to and through the screen andtherefrom toward the electrode 20. The substrate, which here is shown asa roll of paper 24 is pulled by a take-up roll 26, driven by a motor 28,past the printing station. This is defined as the space between twopositioning rollers 30, 32, which guide the paper off the roller 24 andbetween the electrode 20 and the screen 10. The powder image is aflixedto the paper by a fixing device 34. This can comprise a set of heatingcoils.

FIGURE 2 illustrates an embodiment of the invention which may comprisethe powder and delivery device 18' This comprises a hollow housing 40which has a slot open ing 42 in one face thereof. Supportedsubstantially centrally of the housing is a conductive electrode 44. Atthe base of the housing is a rotatably driven brush 46 or other suitablemechanical actuator driven by the motor 48. A source of powder 47,supplies powder, when required, by injecting same over a tube, connectedto the bottom of the housing 40.

The brush is rotatably driven in a clockwise direction. As a result ofits rotation, it causes air to circulate in the direction shown by thearrows. This is, up the region between the electrode 44 and the backsurface of the housing, and thereafter over the electrode and the spacebetween it and the top of the housing and down the front surface of theelectrode to the brush 46. The brush is positioned so that its bristlescome in contact with a supply of electroscopic powder which is in thebottom of the housing. As a result, not only do the rotating brushbristles move air but they also move powder which is thrown upward intothe air and thus forms a powder cloud. Thus the powder cloud is made tocirculate. An external terminal 45 affords the connection of a potentialsource of the electrode 44. Should an electric field be establishedthereby between the electrode 44 and say the screen 10, shown in FIGURE1, the powder in the powder cloud is directed by the field through theslot 42 and onto and through the screen.

The particular embodiment of the invention shown in FIGURE 2 effectivelymoves the air, which is within the chamber, and does not take in any newair or does not require any new air in order to maintain powder cloudcirculation. The result is that the powder cloud is fairly dense, powderbeing added thereto and redisbursed continuously by the rotation of thebrush. A rich air-powder mixture is expelled from the slot 42, as aresult, when a potential is applied between the electrode 44 and anexternal electrode. The dispersion of the powder which is expelled fromthe slot is fairly uniform over the entire area of the slot and providesvery uniform and dense prints. Furthermore, since the air content withinthe chamber is not changed, the powder emitted from the slot iscompletely under the control of the potential applied to the electrodeand is not due to an air flow out of the slot.

It will be appreciated that while the powder delivery device 18 shown inFIGURE 1 delivers the powder directly to the image screen, its utilitymay be, if desired, for delivering the powder to a moving belt or brushto provide a uniform coating thereon. The brush or belt may be usedsubsequently for applying powder to an image electrode.

FIGURE 3 shows another arrangement for supplying a uniform powder supplyover the region of a slot. This time a ring-shaped hollow housing 49 isused. An electrode 50 is positioned in one wall of the housing and inthe wall opposite to the electrode is a slot aperture 52 represented bydotted lines. The housing 49 forms an oval track around which a powderair mixture is moved. A high velocity air source 54 injects air tangentto the outer wall of the housing 49. Powder is injected from a powdersupply 53 into the housing 49. The powder mixes with the air and thepowder-air mixture moves around the track defined by the housing 49.When the mixture passes between the electrode 50 and the slot 52, powdercan be directed outwardly through the slot by applying a potential froma source 56 between the electrode and the external structure towardwhich it is desired the powder to be directed.

Air in sufiicient quantity to avoid air pressure buildup within thehousing is removed from the system, without removing powder, by using arotating porous foam circular structure 58. This is supported so that aportion of its periphery extends into the chamber of the housing 49. Amotor 60 drives the structure 58 so that it rotates in a direction toassist the circulation of the powder-air mixture in the housing. Thatis, assuming the direction of motion of the mixture to be that of thearrows drawn on the housing 49, the Scott foam structure 58 will rotatein a counterclockwise direction. A low pressure air source 62 isconnected through the center support for the rotatable foam structure58.

In operation, the powder is injected into the chamber 49 and movesaround it in the direction indicated by the arrows. As the powder passesbetween the electrode 50 and the slot 52, upon the application of anelectrical potential between the electrode and the outside structureagainst which it is desired to erect the powder, powder comes out of theslot moving in a direction perpendicular to the plane thereof. The lowair pressure source serves to draw excess air out of the chamber 49 andthe foam circular member 58 prevents the powder in the chamber frombeing drawn out with the air. The air must be removed otherwise thepressure in the chamber would soon increase to a value at which it wouldmove powder out of the slot without any potential being applied to theelectrode. The powder which adheres to the surface of the foam pad 58 isthrown off from the periphery of the pad by reason of the rotationthereof. If this does not result in a sulficient release of the powder,the low air pressure source may be interrupted from time to time, or theconnection between the low air pressure source 62 and the center of thepad 58 may be such that the low air pressure source is appliedsuccessively to different portions of the pad.

FIGURE 4 shows another arrangement for circulating a powder-air mixturebetween an electrode and a slot. This arrangement employs a chamber 64,which is similar to the chamber 49. Also, in the upper wall of thechamber is an electrode 66. In the wall opposite the electrode is a slot68 (shown dotted). The source of potential 70 is connected between theelectrode and the object upon which it is desired to transfer powderfrom the powder cloud within the chamber 64.

A high air pressure source is connected through a pipe 74 so that theair is directed as a high velocity stream tangentially along the innerwall wall of a cylindrical container 76. A pipe 78 which has one openingtangentially adjacent the inner wall of the container 76, at its otherend, opens into the container 64. Air is withdrawn from the chamber bymeans of a low air pressure source 92, which is connected through a pipe84 to the center of the top of container 76. Part of the powder-airmixture which is circulating in container 64 is drawn through an opening86 into container 76. The powder-air mixture rotates rapidly withincontainer 76. Powder, being the high er density component of themixture, concentrates near the periphery of the chamber due tocentrifugal force. This concentrated mixture will spiral down the wallof container 76 and be directed by pipe 78 into chamber 64. Thecentrifugal pressure differential in container 76 causes the pressurenear opening 86 to be less than at the opening to pipe 78. The air flowsin pipes 74, 82, 84 are adjusted so that the pressure in container 64 isbetween these two pressures, and is low enough that air is not expelledthrough slot 68. The high air pressure source 80 is directed by pipe 82into container 64 so as to cause the air-powder mixture to circulatearound chamber 64. Powder may be added to chamber 64 from a powdersource by means of a pipe 91.

There has been described and shown herein a novel and useful arrangementfor passing a cloud of electroscopic powder past a slot, with provisionbeing made for directing the powder through the slot and onto areceiving object, upon demand. This arrangement for delivering powderuniformly and with the required density, finds use in situations where amoving belt, or a moving screen must be coated, or where an optimumpowder delivery is desired over a narrow region for nip printing oncurved surfaces.

What is claimed is:

1. An electroscopic powder delivery system comprising walls defining aclosed chamber, a slot opening in one of the walls of said closedchamber, an electrode positioned spaced from said slot, anair-electroscopic powder mixture in said chamber, means for circulatingsaid airelectroscopic powder mixture around said chamber and betweensaid electrode and said slot, and means for applying a potential to saidelectrode when it is desired to direct electroscopic powder out of saidchamber through said slot.

2. Apparatus as recited in claim 1 wherein said electrode extends todivide said chamber into two parts, there being a space between oppositeends of said electrode and adjacent walls of said chamber, a supply ofelectroscopic powder being deposited at one of said ends of saidchamber, and said means for circulating an air-powder mixture aroundsaid chamber between said electrode and said slot including rotatablysupported brush means in contact with said supply of powder, within saidchamber, and means for rotatably driving said brush means.

3. Apparatus as recited in claim 1 wherein said chamber is shaped as avring, said means for driving said airpowder mixture including a sourceof air under pressure, means for directing air from said sourcetangentially along an outer wall of said ring-shaped chamber, and thereis included means for withdrawing air from said chamber.

4. Apparatus as recited in claim 3 wherein said means for Withdrawingair from said chamber comprises porous foam means, means for rotatablysupporting said porous foam meanswith one end extending Within saidchamber, and means for applying low air pressure to the center of saidporous foam means for withdrawing air from said chamber while leavingthe powder particles within said chamber.

5. Apparatus as recited in claim 3 wherein said means for withdrawingair from said chamber comprises a cylindrical container having one endadjacent one of the walls of said chamber, means defining an openingthrough one end of said cylindrical container and said one wall of saidchamber, and means for applying low pressure air to the end of saidcylindrical container which is opposite to the end having an openingtherein.

6. Apparatus for delivering electroscopic powder to a receiving devicecomprising walls defining a hollow chamher, a slot shaped opening in oneof said walls, a conductive partition supported in the center of saidchamber opposite said slot, said partition extending for dividing saidchamber into two parts with access between the two parts being affordedat opposite ends of said partition, a supply of electroscopic powderparticles at one end of said chamber, rotatable brush means supportedrotatably at said one end of said chamber and in contact with saidelectroscopic powder particles, means for rotatably driving saidrotatable brush means for circulating a powderair mixture around saidchamber and between said partition and said slot, and means for applyinga potential to said partition when it is desired to remove powder fromsaid chamber through said slot.

7. A powder delivery system comprising walls defining a hollowring-shaped chamber, a slot in one wall of said chamber, an electrode inthe wall of said chamber opposite to the wall having said slot, meansfor introducing air at a pressure in excess of atmospheric pressure intosaid chamber to circulate around said ring-shaped chamber, means forintroducing electroscopic powder into said chamber to be carried thereinby said air, means for removing excess air from said chamber, and meansfor applying a potential to said electrode when it is desired to removepowder from said chamber through said slot.

8. Apparatus as recited in claim 7 wherein said means for introducingair at a pressure in excess of atmospheric into said chamber tocirculate around said chamber includes a hollow container having one endpositioned adjacent one wall of said ring-shaped container, a source ofair at a pressure inexcess of atmospheric pressure, means for directingair from said source tangentially at one end of said cylindricalcontainer, means for directing air from the other end of saidcylindrical container into said ring-shaped container, means forintroducing air under pressure tangentially along a wall of saidringshaped container for circulating said powder-air mixture, wallsdefining an opening extending between the end of said cylindricalcontainer adjacent said one wall of said ring-shaped container and intoand through said one wall of said ring-shaped container, source of airat a pressure less than atmospheric, and means for coupling said sourceof air at a pressure less than atmospheric to the interior of saidcylindrical container through the other end thereof.

References Cited UNITED STATES PATENTS 2,928,575 3/1960 Carlson 222-1932,935,234 2/1960 Huber 222193 3,221,938 12/1965 Yonkers et al 22276ROBERT E. REEVES, Primary Examiner H. S. LANE, Assistant Examiner US.Cl. X.R. 222l93; 118637

